Atmospheric discharge plasma in aqueous solution: Importance of the generation of water vapor bubbles for plasma onset and physicochemical evolution

Discharge plasma formed in aqueous solutions has attracted much attention for its applications in environmental purification and material syntheses. The onset and evolution of the discharge plasma in an aqueous solution and transient reactive species formed in it are successfully monitored with micrometer spatial resolution and nanosecond temporal resolution. The combination of a custom-made microscopic discharge system and a high-speed camera provides direct evidence that water vapor bubbles form before the discharge with the thermal phase transition of aqueous solution at the electrode tip. The water vapor bubbles, i.e., locally formed space in the gas phase, connect the gap between the tips of the opposed electrodes. The local gas area formed in aqueous solution plays a crucial role in the ignition and continuance of the discharge plasma. It is also found that the initially formed plasma lasts for under 100 ns and quenches rapidly. However, plasma regenerates in the water vapor bubble and successively bridges the opposing electrodes during the pulsed-voltage application (ca. 1 mu s). These two temporally distinct generations of plasma, i.e., the initial plasma (IP) and the following successive plasma (SP), can be seen to correspond to the dielectric breakdown and glow-like plasma, respectively. These results provide an important picture for the proposed mechanism for plasma evolution in water and also important information for the efficient control of the discharge plasma with its applications in waste-water treatments, nanomaterial syntheses with plasma oxidation-reduction reactions, and the chemical modification of the material surfaces in aqueous solutions as a form of green chemistry. Published by AIP Publishing.